I am studying grammar at level B1/B2 German and I encountered something I do not really understand: At the moment I am working on a chapter concerning prepositions which require the dative. In this ...

German is not my native language but I have a lot of experience with it. To translate "because of what he said", I feel like I should say "wegen dem, was er gesagt hat" instead of "wegen dessen, was ...

In this question we learned that one origin of new prepositions are adverbs or fixed prepositional phrases. These prepositions go initially with genitive (and then perhaps mutate into dative).
When ...

Wiktionary lists 2 genitive forms for a lot of nouns (e.g. Berg, Brauch), -es and -s.
For these words and others, are these forms interchangeable or are they regional? Or is one more poetic sounding ...